Versatile system for output energy limiting circuitry

ABSTRACT

The present invention provides a system for limiting energy levels across the output of a driver circuitry segment ( 100 ). The system provides an output structure ( 102 ) adapted to drive an output load ( 104 ). A transconductance component ( 106 ) is communicatively coupled to the output structure, and adapted to output a transconductance current that is proportional to the voltage across the output structure. A scaling component ( 108 ) is communicatively coupled to the output structure, and adapted to output a scaled current that is proportional, by some scaling factor, to the current through the output structure. A qualifying component ( 110 ) is communicatively coupled to the scaling component, and adapted to activate a trigger component ( 112 ) when the scaled current passes a first threshold. The trigger component is communicatively coupled to the qualifying component, the transconductance component, and the output structure. Responsive to activation from the qualifying component, the trigger component receives the transconductance current and accumulates charge, for an amount of time inversely proportional to the transconductance current&#39;s magnitude, and triggers shut off of the output structure when the accumulated charge passes a second threshold.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to the field of semiconductordevices and, more particularly, to a versatile system of apparatus andmethods for limiting energy overload across driver output circuitry.

BACKGROUND OF THE INVENTION

The continual demand for enhanced integrated circuit performance hasresulted in, among other things, a dramatic reduction of semiconductordevice geometries, and continual efforts to optimize the performance ofevery substructure within any semiconductor device. A number ofimprovements and innovations in fabrication processes, materialcomposition, and layout of the active circuit levels of a semiconductordevice have resulted in very high-density circuit designs. Increasinglydense circuit design has not only improved a number of performancecharacteristics, it has also increased the importance of, and attentionto, semiconductor material properties and behaviors.

The increased packing density of the integrated circuit generatesnumerous challenges to the semiconductor manufacturing process. Nearlyevery device must be smaller without degrading operational performanceof the integrated circuitry. High packing density, low heat generation,and low power consumption, with good reliability must be maintainedwithout any functional degradation. Increased packing density ofintegrated circuits is usually accompanied by smaller feature size and,correspondingly, smaller device geometries.

At the same time, the use of electronic products and systems has spreadinto a number of new and distinct applications—many of which were not,until recently, associated with electronic technology. Often, such newapplications place a number of unique demands on circuitry componentsand substructures. Consider, for example, the radiation tolerancerequired of satellite or spacecraft systems, or the heat and shocktolerance required of automotive systems.

Thus, optimized performance over a broader range of operating conditionsis required of many electronic components and substructures. This hasresulted in a number of improvements and innovations in electronicsystems, and has increased the importance of, and attention to,component and substructure properties and behaviors.

Commonly, system designers specify or define a number of requiredoperational parameters (e.g., max/min voltage, signal timing) forcertain circuitry segments in a system. Semiconductor devices (i.e.,integrated circuits) must comply with such required parameters in orderto be used in the system. For example, a system may require that asemiconductor device operate over supply voltage range of 0V to 40V,optimized for performance at 20V. In another example, a system mayrequire that a semiconductor device provide a specified timing parameter(e.g., t_(rise(MIN)), t_(fall(MAX))).

Unfortunately, however, there are a large number of variables insemiconductor device manufacturing that can affect any given performanceparameter. Intra-process variations, feature matching issues, and layoutconsiderations are among a number of concerns that impact a devicemanufacturer's ability to provide a specified performance parameter. Insome cases, a semiconductor device's standard operational parameters maybe sufficient to provide a required performance level in a given system.In a number of other cases, however, a given system may require a veryspecific or peculiar performance parameter—such that an integratedcircuit must be designed specifically for that application, if possible.

Consider, for example, a common driver circuit, such as an amplifier,utilized in a high-voltage application that has specific performanceparameters. Commonly, performance specifications require that drivercircuitry consume relatively low power while driving a significant load.In a number of applications, an output node from a driver circuit can becoupled to a load having a wide variety of “normal” operatingconditions—such as relatively large voltage swings during standardoperations. Components and structures within the driver circuit maytherefore be adapted to tune the circuit to a required range ofoperating conditions. Depending upon the design and fabricationprocesses used, however, certain adapted circuitry components may besusceptible to performance degradation or break down if non-standardconditions (e.g., overload, short) cause output circuitry to exceed thestandard operating range.

For example, sustained power overload on certain output circuitry cancause a significant rise in operating temperature, which can—overtime—begin to break down transistor structures. Even over a relativelyshort amount of time, an excessive output energy drop can degrade theperformance of output transistor structures, or render them completelyinoperable. In addition, increased energy drop across a driver outputcan significantly increase overall system power consumption. This cancause a number of system inefficiency or reliability issues, or causesignificant process yield problems for the device manufacturer.

As a result, a number of protection schemes have been implemented in anattempt to prevent overloading of such output circuitry. Generally,protection schemes rely on some form of all-or-nothing overcurrentprotection—implemented as circuitry that shuts down the driver outputcomponent(s) every time output current level(s) exceed some arbitrarythreshold. Often, this threshold is set at some value within, by atleast a nominal margin, the specified operating range of the drivercircuitry. Using such schemes, even instantaneous variations in currentthat exceed the threshold cause output shutdown—even though suchvariations may still be marginally within the operational capability ofthe output components, or may last for such a brief period that nodamage would actually accrue to the output components if left unchecked.In certain system applications, and especially in signal-intensiveapplications, minor overcurrent variations (i.e., those that would nototherwise cause damage to an output structure) occur randomly andfrequently. Utilizing conventional output protection schemes in suchapplications could result in a potentially high degree of systeminefficiency or cause system malfunction if the signal integrity issufficiently degraded, as driver output components are repeatedlycycling off and on every time an arbitrary threshold is exceeded.

Other conventional protection schemes err on the side of lesscomprehensive protection—choosing instead to set protective thresholdswell outside of the specified operating range of driver circuitry. Suchapproaches typically provide a protective cutoff or shutoff only in thecase of some catastrophic overload condition. These under-correctiveschemes typically do not trigger for overloads that are only marginallyoutside of the specified operating range of driver circuitry, even ifsuch overloads occur for an extended length of time. These schemes,therefore, are quite capable of permitting extensive damage to outputstructures, over time.

As a result, there is a need for a system for protective driver outputcircuitry or structures that effectively limit output energy levelswithout over or under correcting, thereby reducing device and systeminefficiencies or malfunctions introduced by excessive or restrainedoff-cycling of device output structures—one that is readily adaptable toaddress a variety of specific parametric requirements, while providingefficient and reliable device performance in an easy, efficient andcost-effective manner.

SUMMARY OF THE INVENTION

The present invention provides a versatile system, comprising variousapparatus and methods, for limiting output energy levels acrosshigh-voltage driver circuitry, without over or under correction. Thesystem of the present invention thereby reduces device and systeminefficiencies commonly associated with conventional systems' excessiveor inadequate off cycling of device output structures. The system of thepresent invention comprehends, and is readily adaptable to address, awide variety of specific parametric requirements imposed by specificend-equipment applications. The present invention thus providesefficient and reliable device performance in an easy, cost-effectivemanner.

Specifically, the present invention provides circuitry that protectsdriver output structures (e.g., transistors) from exposure to excessiveoutput energy levels that might otherwise damage or destroy those outputstructures. The circuitry of the present invention dynamically monitorsboth current and voltage levels across output structures to determinewhether those output structures should be shut off to avoid exposure toexcessive energy levels. The present invention recognizes that theenergy across any given structure is a function of current, voltage andtime. As an output structure experiences an overcurrent condition, theduration of the condition, and the magnitude of voltage across theoutput structure during the condition, determine whether the outputstructure is shut off. If an overcurrent condition exists, and thevoltage across the output structure is relatively high, the outputstructure is shut off in a much shorter time than if the voltage acrossthe output structure is relatively low. In such a manner, the presentinvention provides a dynamic deglitch time, which optimizes protectionof output structures to a specific energy level limit, not just aselected current or voltage level threshold. This provides robustprotection while efficiently accommodating temporary signal variancesthat commonly occur.

More specifically, certain embodiments of the present invention providea system for limiting energy levels across a driver circuitry outputstructure that is adapted to drive an output load. A transconductancecomponent is communicatively coupled to the output structure, andadapted to output a transconductance current that is proportional to thevoltage across the output structure. A scaling component iscommunicatively coupled to the output structure, and adapted to output ascaled current that is proportional, by some scaling factor, to thecurrent through the output structure. A qualifying component iscommunicatively coupled to the scaling component, and adapted toactivate a trigger component when the scaled current passes a firstthreshold. The trigger component is communicatively coupled to thequalifying component, the transconductance component, and the outputstructure. Responsive to activation from the qualifying component, thetrigger component receives the transconductance current and accumulatescharge, for an amount of time inversely proportional to thetransconductance current's magnitude, and triggers shut off of theoutput structure when the accumulated charge passes a second threshold.

Other embodiments of the present invention provide a method of limitingenergy levels across a driver circuitry output structure. Current levelthrough the output structure is monitored and compared to a firstthreshold value. When the current level across the output structurepasses the first threshold, a charge proportional to a voltage levelacross the output structure is accumulated, and compared to a secondthreshold value. The output structure is shut off when the accumulatedcharge passes the second threshold.

Certain embodiments of the present invention further provide circuitryhaving first and second voltage supplies and an output node. A firsttransistor has its source coupled to the first voltage supply, its draincoupled to the output node, and its gate coupled to a first node. Asecond transistor has its source coupled to the first voltage supply,its drain coupled to the first node, and its gate coupled to an outputof a first inverter. A third transistor, formed to match the firsttransistor by a scaling factor, has its source coupled to the firstvoltage supply, its drain coupled to a second node, and its gate coupledto the first node. A resistance network, is coupled between the secondnode and the second voltage supply. A first comparator has a first inputcoupled to the second node, a second input coupled to a first thresholdvoltage source, and an output coupled to a third node. A second inverterhas an input coupled to the third node and an output. A fourthtransistor has its source coupled to the second voltage supply, itsdrain coupled to a fourth node, and its gate coupled to the output ofthe second inverter. A second comparator has a first input coupled tothe fourth node, a second input coupled to a second threshold voltagesource, and an output coupled to an input of the first inverter. Acapacitor is coupled between the fourth node and the second voltagesupply. A controlled current source is coupled to the third and fourthnodes. A transconductance component is coupled to the first transistorand to the fourth node, and delivers a current to the capacitor that isproportional to voltage across the first transistor.

Other features and advantages of the present invention will be apparentto those of ordinary skill in the art upon reference to the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show by way ofexample how the same may be carried into effect, reference is now madeto the detailed description of the invention along with the accompanyingfigures in which corresponding numerals in the different figures referto corresponding parts and in which:

FIG. 1 provides an illustration depicting one embodiment of a drivercircuitry design in accordance with certain aspects the presentinvention;

FIG. 2 provides an illustration depicting another embodiment of a drivercircuitry design in accordance with certain aspects the presentinvention; and

FIG. 3 provides an illustration depicting another embodiment of a drivercircuitry design in accordance with certain aspects the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts, whichcan be embodied in a wide variety of specific contexts. The presentinvention is hereafter illustratively described primarily in conjunctionwith the design and operation of driver circuitry in the form anoperational amplifier. Certain aspects of the present invention arefurther detailed in relation to design and operation of circuitryutilizing a PMOS transistor within a CMOS semiconductor process.Although described in relation to such structures, the teachings andembodiments of the present invention may be beneficially implementedwith a variety of semiconductor devices or structures (e.g., NMOStransistors, low/high side driver systems). The specific embodimentsdiscussed herein are, therefore, merely demonstrative of specific waysto make and use the invention and do not limit the scope of theinvention.

The present invention provides a versatile system with which energylevels across a driver circuitry output may be protectively limited,without over-correction or under-correction. The system of the presentinvention thereby reduces device and system inefficiencies ormalfunctions commonly associated with excessive or inadequate offcycling of device output structures. The system of the present inventionis robust—comprehending, and readily adaptable to, a wide variety ofspecific parametric requirements. The present invention thus providesefficient and reliable device performance in an easy, cost-effectivemanner.

According to the present invention, circuitry is provided to protectdriver output structures (e.g., transistors) from exposure to excessiveoutput energy levels—levels that might otherwise damage or destroy thoseoutput structures. The circuitry of the present invention dynamicallymonitors both current and voltage levels across output structures, inorder to determine whether those output structures should be shut off toavoid exposure to excessive energy levels.

The present invention recognizes that the energy across any givenstructure is a function of current, voltage and time. As such, thesystem of the present invention incorporates all three factors inproviding a dynamic deglitch period. When an output structure of concernexperiences an overcurrent condition, a deglitch period begins beforethe output structure is shut off. This deglitch period varies with themagnitude of voltage across the output structure during the overcurrentcondition. According to the present invention, if an overcurrentcondition exists, and the voltage across the output structure isrelatively high, the output structure is shut off in a much shorter timethan if the voltage across the output structure is relatively low.Protection of output structures is thus optimizable to a specific energylevel limit, not just an arbitrary current level threshold. The presentinvention thus provides stable circuitry protection while efficientlyaccommodating temporary signal variances that commonly occur.

According to the present invention, a number of components or subsystemsinteroperably address separate performance or design issues within thedriver circuitry. Certain aspects of components and subsystems accordingto the present inventions are described now with reference to drivercircuitry system 100, as depicted in FIG. 1. System 100 comprises drivercircuitry having an output structure 102. Output structure 102 maycomprise one or more transistors arranged in an amplifier or driverconfiguration, adapted to drive an output load 104. A transconductancecomponent 106 is communicatively coupled to structure 102, havingconnections therewith sufficient to measure voltage across the outputstructure, and adapted to output a current that is proportional to thatvoltage.

Structure 102 is further communicatively coupled to scaling component108, which scales the current across structure 102 by some desiredscaling factor (Z). Component 108 provides this proportionally scaledcurrent to a qualifying component 110. When component 110 determinesthat the proportionally scaled current has crossed a desired threshold,it activates a trigger component 112. Trigger component 112 receives asignal from component 106 that is proportional to the voltage acrossstructure 102. For an amount of time, inversely proportional to themagnitude of the signal received from component 106, component 112charges until crossing some predetermined threshold. Once this thresholdhas been crossed, component 112 triggers a signal that switchescomponent 102 off.

Thus, by the present invention, the energy level across structure 102 iseffectively limited. The boundaries of this limitation may be altered oradjusted by varying threshold or proportion levels within system 100. Inits protection of structure 102, system 100 comprehends not onlyexcessive current levels, but also comprehends voltage levels during,and duration of, the excessive current levels—providing an efficient andproportionate measure of output energy level. This system is readilyadaptable to a number of circuit topologies and device types.

Further details regarding certain aspects of the present invention areprovided now with reference to driver circuitry system 200, as depictedin FIG. 2. System 200 comprises driver circuitry having first and secondoutput structures 202 and 204. First output structure 202 comprises aPMOS transistor having its source coupled to supply voltage 206, itsdrain coupled to output node 208, and its gate coupled to node 210.Second output structure 204 comprises an NMOS transistor having its gatecoupled to node 212, its drain coupled to node 208, and its sourcecoupled to ground. The circuitry of system 200 drives a load 214, whichis coupled to node 208.

System 200 further comprises a first switching component 216 is coupledto node 210. Component 216 comprises a PMOS transistor having its sourcecoupled to supply voltage 206, its drain coupled to node 210, and itsgate coupled to the output of an inverter 218. A secondary PMOStransistor 220 has its source coupled to supply voltage 206, its draincoupled to node 222, and its gate coupled to node 210. A firstprotection resistor 224 is coupled between node 222 and node 226. Asecond protection resistor 228 is coupled between node 226 and ground. Afirst comparator 230 has a first input coupled to node 222. A firstthreshold voltage source 232 is coupled between the second input ofcomparator 230 and ground. The output of comparator 230 is coupled tonode 234. Node 234 is also coupled to the input of an inverter 236 andthe gate of a second PMOS switching transistor 238. The output ofinverter 236 is coupled to shunt component 240.

Component 240 comprises an NMOS transistor having its gate coupled toinverter 236, its drain coupled to node 242, and its source coupled toground. Node 242 is also coupled to a charging component 244, to a firstinput of comparator 246, to the drain of first charging transistor 248,and to the output of a transconductance component 250. Chargingcomponent 244 comprises, in this embodiment, a capacitor or capacitivenetwork coupled between node 242 and ground. Component 250 comprises atransconductance amplifier having a first input coupled to the source oftransistor 202, and a second input coupled to node 208. First chargingtransistor 248 is in a current mirror configuration with second chargingtransistor 252. The sources of transistors 238, 248 and 252 arecollectively coupled together. The gates of transistor 248 and 252 arecoupled together at node 254, which is also coupled to the drain oftransistor 238. Node 254 is coupled to the drain of transistor 252, aswell as to current source 256.

Functionally, the driver circuitry portion of system 200 operates as alinear amplifier, operating between two power supplies (i.e., supply 206and ground). This amplifier delivers an output signal (e.g., V_(OUT)) toload 214. Depending upon the configuration and operation of load 214,the load might sink or source current through node 208. The drivercircuitry and, particularly, transistor 202 are designed to handle somemaximum level of alternating current (i.e., peak current (I_(PEAK)))through node 208. For example, assume that I_(PEAK) for standard ACoperation in system 200 is 0.5 A (or 500 mA).

Depending upon the nature of load 214, certain non-standard operatingconditions (e.g., overload, short) could cause, for example, a sustaineddirect current (DC) of 0.5 A at node 208. Without the aid of the presentinvention, the constituent structures of transistor 202—not designed forsustained 0.5 A DC—would rapidly begin to heat up as the energy levelacross transistor 202 increases. Once the temperature rises to acritical threshold (e.g., ˜250° C.) for certain semiconductor materialswithin the transistor, structures formed of those materials begin toaccrue damage and break down. Utilizing the present invention, however,energy accumulation across transistor 202 is limited, and theaforementioned damage is obviated.

According to the present invention, transistor 220 is a relatively smalltransistor—matched to transistor 202 but formed of a size that is somesmall fraction (e.g., 1/50^(th), 1/100^(th)) of the size of transistor202. Transistor 220 is configured with transistor 202 in a currentmirror construct. Thus, at any given time, if the current throughtransistor 202 is (I₂₀₂), then the current across transistor 220 (I₂₂₀)is a proportional fraction thereof (e.g., I₂₀₂/100). Based on thedesired design and performance characteristics of system 200, the peakcurrent (I_(PEAK)) allowable across transistor 202, before damageoccurs, is determined. This sets a threshold that, in part, triggers theprotection circuitry of the present invention, as described hereinafter.Correspondingly, peak current allowable across transistor 220 becomes aproportional fraction thereof (e.g., I_(PEAK)/100).

A resistor network 256, comprising resistors 224 and 228, is disposedbetween the drain of transistor 220 and ground. The values andconfiguration of the resistors in network 256 are configured to providea desired trigger voltage at node 222. For example, if I_(PEAK) fortransistor 202 is 500 mA, then I_(PEAK) for transistor 220 is 5 mA. Ifthe desired trigger voltage at node 222 is, for example, 5V, then thetotal effective resistance of network 258, as realized at node 222, mustbe 1 kΩ (5 mA×1 kΩ=5V). Threshold voltage source 232 provides a desiredtrigger voltage level to comparator 230. Thereby, once I₂₀₂ has exceededI_(PEAK), the voltage level at node 222 exceeds the desired triggervoltage level—triggering comparator 230. Thus, the values of source 232and network 258 may be varied across devices or designs to adapt system200 to the requirements of a particular application.

When comparator 230 triggers, it outputs a trigger signal through node234. That trigger signal propagates through inverter 236 to shut offtransistor 240. In standard operating mode (i.e., I₂₀₂>I_(PEAK)),transistor 240 is active and shunts current through node 242 to ground,keeping capacitor 244 from charging. Once a trigger signal deactivatestransistor 240, however, capacitor 244 charges from two sources—theoutput of transconductance amplifier 250, and a current source structure260. The voltage across capacitor 244, as measured at node 242, isconducted to comparator 246. When the voltage across capacitor 244exceeds a desired voltage threshold, as provided by a second thresholdvoltage source 262, comparator 246 triggers—sending a trigger signal,through inverter 218, to switch transistor 216. This activatestransistor 216, which shuts off transistor 202 by clamping its gate toits source.

Shut off of transistor 202 is thereby based upon the triggering ofcomparator 246. The triggering of comparator 246 is based on the voltagelevels of source 262 and capacitor 244. Source 262 comprises anysuitable voltage source that provides a desired voltage thresholdreference level. As described hereinafter, the voltage across capacitor244 and its rate of charge serve as an indicator of the energy levelacross output structure 202. The threshold value of source 262 isselected such that as soon as the voltage across capacitor 244 reaches alevel indicating an excessive energy level at the output structure 202,that output structure is shut off. Thus, the value of source 262 may bevaried across devices or designs to adapt system 200 to the requirementsof a particular application.

As described, charging component 244 effectively integrates the poweracross structure 202 over time, providing an indication of the energylevel thereof. Component 244 does not charge unless I₂₀₂ has exceededI_(PEAK), which means an overcurrent condition exists. Once anovercurrent condition exists, component 244 is allowed to charge. Itdraws current from current source structure 260 and fromtransconductance component 250.

Current source structure 260 comprises a current mirror structurecomprising transistors 248 and 252, and controlled current source 256.Structure 260 is switched on or off by transistor 238, responsive to atrigger signal from comparator 230. When active, structure 260 providesa “default” current level to component 244—one that charges component244 at some nominal, fixed rate. Depending upon the requirements of aspecific design or application, the characteristics (e.g., magnitude) ofsource 256 may be altered to accommodate those requirements.

Transconductance component 250 comprises a transconductance amplifierthat translates, as described hereinafter, the drain/source voltage(V_(DS)) of transistor 202 into a proportionate current level. During anovercurrent condition, component 250 charges component 244 withincreasing current as the voltage level across transistor 202 increases.Thus, the greater the voltage across transistor 202, the fastercomponent 250 charges component 244. This triggers comparator 246faster, switching transistor 216 earlier to avoid prolonged exposure oftransistor 202 to the increased voltage condition. Similarly, arelatively low voltage across transistor 202 will drive a lower currentfrom component 250—charging component 244 slower and increasing theamount of time before shut off of transistor 202 is triggered. Thesystem of the present invention thus dynamically adjusts—based on thepower level across transistor 202—the length of time that transistor 202is exposed to that power level, limiting the energy level across thatoutput structure.

Transconductance component 250 may be implemented in a number of ways,so long as the component delivers some charge dynamically proportionalto the voltage level across the output structure. One illustrativeembodiment of a transconductance component is described now withreference to circuitry segment 300 of FIG. 3. Segment 300 depicts afunctional representation of a transconductance component that providesseparate output current proportional to voltage across each of twooutput transistors 302 and 304. Transistors 302 and 304 comprise anamplifier output stage driving an output 306, similar to transistors 202and 204. Transistor 302 is coupled between output 306 and a first supplyvoltage 308 (V_(S)). Transistor 304 is coupled between output 306 and asecond supply voltage (e.g., ground). A mirror transistor 310 has itsgate coupled to output 306, its source coupled to a resistor 312, andits drain coupled to node 314. Resistor 312 is coupled between thesource of transistor 310 and the supply voltage 308. Assuming that, fora high voltage application, the gate to source voltage for transistor310 is small compared to the voltages across output devices 302 and 304,the current 316 realized between the drain of transistor 310 and node314 is approximately equal to:(V_(S)−V_(OUT))/R₃₁₂   (1)which varies proportionately with:V_(DS302)/R₃₁₂   (2)Another mirror structure, comprising transistors 318 and 320, is alsocoupled to node 314. The gates of transistors 318 and 320 are jointlycoupled to node 314. Current 322—the current through transistor 320—iseffectively the same as current 316, which is proportional to thevoltage across transistor 302 (V_(DS302)). Transistor 320 thus outputs acurrent 322 that is proportional to V_(DS302). A second mirrortransistor 324 has its gate also coupled to node 314. The source oftransistor 324 is coupled to ground, and its drain is coupled to node326. Current 328 (I₃₂₈) flows through transistor 324 and is, similar tocurrent 322, also proportional to V_(DS302).

A resistor 330 is coupled between node 326 and supply voltage 308.Resistor 330 has the same resistance as resistor 312. Thus, current 332(I₃₃₂) is proportional to (V_(S)/R₃₃₀). Thus, current 334 (I₃₃₄) at node326 is effectively equivalent to (I₃₃₂-I₃₂₈). The voltage acrosstransistor 304 (V_(DS304)) is equivalent to V_(OUT), which is equivalentto (V_(S)-V_(DS302)). I₃₃₂ less the current that is proportional to thevoltage across transistor 302 (i.e., I₃₂₈) should render a current thatis proportional to the voltage across transistor 304. That currentdifferential is I₃₃₄, which is therefore proportional to the voltageacross transistor 304 (V_(DS304)).

Thus, transconductance component 300 provides two useful outputs—acurrent proportional to the voltage across the high side outputtransistor, and a current proportional to the voltage across the lowside output transistor. Although presented for purposes of explanationand illustration, segment 300 is a simplified circuit that may, indesign, be implemented in a number of ways. In certain applications, forexample, circuitry may be implemented to account for the output voltagebeing in a high impedance (or other unknown) state. For example,circuitry within a transconductance component may be implementedutilizing diode-connected transistors, for which current and voltagecharacteristics are predefined. Since current delivered from a voltagesupply is also predefined, knowledge of the voltage at a node betweentwo output structures is not critical. Currents proportional to thevoltages across each of those output structures may be provided withoutreliance on a known V_(OUT). Other devices and structures may beimplemented within a transconductance component to, for example, cycleoff or on certain portions of circuitry, providing different modes ofoperation during which different voltage and current values are beingdetermined. These and other similar variations are comprehended by thepresent invention.

The present invention thus provides a very versatile system for limitingexcessive energy levels across certain output structures—energy levelsthat would otherwise damage semiconductor structures and features withinthose output structures. The system of the present invention may,however, be optimized to obviate inefficiencies associated withsingle-threshold circuitry shutoff schemes. The system of the presentinvention comprehends not only excessive current levels across an outputstructure, but also voltage levels corresponding to and time of suchexcess. By the present invention, robust driver circuitry may beefficiently produced in commercially viable semiconductor processtechnologies while still comprehending specific parametric requirementsand restrictions imposed by a particular application. The presentinvention thus addresses issues of concern in a commercially viablemanner.

The embodiments and examples set forth herein are therefore presented tobest explain the present invention and its practical application, and tothereby enable those skilled in the art to make and utilize theinvention. However, those skilled in the art will recognize that theforegoing description and examples have been presented for the purposeof illustration and example only. For example, certain aspects of thepresent invention have been described above in relation to PMOStransistor structures. The teachings and principles of the presentinvention are equally applicable, however, to NMOS transistorstructures. The description as set forth herein is therefore notintended to be exhaustive or to limit the invention to the precise formdisclosed. As stated throughout, many modifications and variations arepossible in light of the above teaching without departing from thespirit and scope of the following claims.

1. A driver circuitry segment comprising: an output structure adapted todrive an output load; a transconductance component, communicativelycoupled to the output structure and adapted to output a first currentthat is proportional to a first voltage across the output structure; ascaling component, communicatively coupled to the output structure andadapted to output a second current that is proportional to a thirdcurrent across the output structure by some scaling factor; a qualifyingcomponent, communicatively coupled to the scaling component and adaptedto activate a trigger component when the second current passes a firstthreshold; and a trigger component, communicatively coupled to thequalifying component, the transconductance component, and the outputstructure; wherein the trigger component, responsive to activation fromthe qualifying component, receives the first current from thetransconductance component and accumulates charge, for an amount of timeinversely proportional to the first current's magnitude, and triggersoutput structure shut off when the accumulated charge passes a secondthreshold.
 2. The circuitry segment of claim 1, wherein the outputstructure comprises a PMOS transistor.
 3. The circuitry segment of claim1, wherein the output structure comprises a NMOS transistor.
 4. Thecircuitry segment of claim 1, wherein the scaling component comprises atransistor that is formed, in relation to a transistor in the outputstructure, of a size corresponding to the scaling factor.
 5. Thecircuitry segment of claim 1, wherein the qualifying component comprisesa comparator adapted to compare a second voltage, corresponding to thesecond current across a known resistance, to a first voltage thresholdsupplied by an external source.
 6. The circuitry segment of claim 1,wherein the trigger component comprises a capacitor adapted toaccumulate charge from the first current responsive to activation fromthe qualifying component.
 7. The circuitry segment of claim 6, whereinthe trigger component further comprises a comparator adapted to comparevoltage across the capacitor to a second voltage threshold supplied byan external source.
 8. The circuitry segment of claim 6, wherein thetrigger component further comprises a controlled current source adaptedto supply current to the capacitor responsive to activation from thequalifying component.
 9. The circuitry segment of claim 1, wherein theoutput structure further comprises a high side PMOS transistor and a lowside NMOS transistor.
 10. The circuitry segment of claim 9, wherein thetransconductance component further comprises: a first current structureadapted to output a current proportional to voltage across the high sidePMOS transistor; a second current structure adapted to output a currentproportional to voltage across the low side NMOS transistor; andcircuitry adapted to selectively output current from the first currentstructure or the second current structure as the first current.
 11. Amethod of limiting energy level across a driver circuitry outputstructure, the method comprising the steps of: monitoring a currentlevel across the output structure; comparing the current level acrossthe output structure to a first threshold; when the current level acrossthe output structure passes the first threshold, accumulating a chargeproportional to a voltage level across the output structure; comparingaccumulated charge to a second threshold; and shutting off the outputstructure when the accumulated charge passes the second threshold. 12.The method of claim 11, wherein the step of accumulating a chargeproportional to a voltage level across the output structure furthercomprises accumulating the charge in an amount of time that is inverselyproportional to the magnitude of the voltage level across the outputstructure.
 13. The method of claim 12, wherein the step of accumulatinga charge proportional to a voltage level across the output structurefurther comprises: providing a transconductance component adapted tooutput a transconductance current that is proportional to the voltagelevel across the output structure; providing a capacitor; and chargingthe capacitor with the transconductance current when the current levelacross the output structure passes the first threshold.
 14. The methodof claim 11, wherein the step of monitoring a current level andcomparing the current level further comprise: providing a scalingcomponent adapted to output a scaled current that is proportional to thecurrent level through the output structure; passing the scaled currentthrough a known resistance; and comparing voltage across the knownresistance to the first threshold.
 15. The method of claim 13, whereinthe step of accumulating a charge further comprises: providing acontrolled current source adapted to supply a fixed current; andcharging the capacitor with the fixed current when the current levelacross the output structure passes the first threshold.
 16. The methodof claim 11, wherein the step of shutting off the output structurefurther comprises: providing a switch component coupled the outputstructure; and activating the switch component responsive to theaccumulated charge passing the second threshold.
 17. A circuitcomprising: first and second voltage supplies; an output node; a firsttransistor having its source coupled to the first voltage supply, itsdrain coupled to the output node, and its gate coupled to a first node;a second transistor having its source coupled to the first voltagesupply, its drain coupled to the first node, and its gate coupled to anoutput of a first inverter; a third transistor, formed to match thefirst transistor by a scaling factor, having its source coupled to thefirst voltage supply, its drain coupled to a second node, and its gatecoupled to the first node; a resistance network, coupled between thesecond node and the second voltage supply; a first comparator, having afirst input coupled to the second node, a second input coupled to afirst threshold voltage source, and an output coupled to a third node; asecond inverter having an input coupled to the third node and an output;a fourth transistor having its source coupled to the second voltagesupply, its drain coupled to a fourth node, and its gate coupled to theoutput of the second inverter; a second comparator, having a first inputcoupled to the fourth node, a second input coupled to a second thresholdvoltage source, and an output coupled to an input of the first inverter;a capacitor coupled between the fourth node and the second voltagesupply; a controlled current source, coupled to the third and fourthnodes; and a transconductance component, coupled to the first transistorand to the fourth node, and adapted to deliver a current to thecapacitor that is proportional to voltage across the first transistor.